This invention relates to a pressure sensor of the type having a sensing fiber subjected to microbends at a material boundary in response to a pressure to be sensed, the sensing fiber comprising a fiber of a transparent medium surrounded by a layer of another medium. It further relates to such a pressure sensor having a light source providing a known light at one end of the transparent fiber and means to sense the light transmitted through the fiber, the latter being reduced by microbend induced losses at the boundary and therefore varying with the sensed pressure.
In the prior art, such pressure sensors are well known using optic communication fibers in which a central core of transparent glass is surrounded by a cladding of glass having an index of refraction less than that of the core. The cladding is generally surrounded by a jacket of light absorbing and physically protective material, an example of which is aluminum. Light is coupled into one end of the core and is reflected from the core/cladding boundary to remain within the core except where the core, cladding and boundary therebetween are affected by microbends applied to the outer surface of the fiber. The microbends affect the core/cladding boundary and the fields within the core and cladding so that a portion of the light in the core escapes through the boundary into the cladding and is eventually absorbed by the jacket surrounding the cladding. The phenomenon is generally explained in simple terms of microbend induced changes in the angle of incidence of the light rays in the core at the boundary leading to a greater portion of the rays passing through the boundary rather than being reflected, although the actual physical processes involved appear to be more complicated, involving the fields of the individual modes within core and cladding.
One parameter of a microbending fiber optic pressure sensor which is of importance is its sensitivity. Prior art sensors have generally used communication grade fibers of the multimode type, in which the core is capable of carrying a plurality of propagation modes, primarily because they are less expensive and easier to work with than single mode fibers. Communication grade fibers, however, are designed for minimal sensitivity to external conditions; and this works against sensitivity to microbending. The sensitivity of sensors using multimode fibers is marginal or insufficient for some applications, such as use as a combustion pressure sensor in an internal combustion engine spark plug. Recent study has indicated that certain single mode fibers, with careful setup and accurate control of dimensions, periodicity of microbending, etc. can exhibit sensitivity somewhat greater than that of the commonly available multimode fibers. However, the increase in sensitivity is still not sufficient for some applications; and the expense of single mode fibers and the careful control of all parameters necessary to obtain the higher sensitivity are handicaps to the use of single mode fibers in this manner.